The present invention relates to a method for controlling a simulated moving bed system. More particularly, the invention relates to a method for minimizing the peak pressure of a simulated moving bed system by using variable speed centrifugal pumps.
A simulated moving bed system is known for chromatographic separation, which system is extensively used in industry for separating hydrocarbon or saccharide mixtures.
A simulated moving bed system generally consists of several beds each packed with a solid adsorbent, with the downstream end of each bed and the upstream end of the next succeeding bed being connected by conduits to form an endless loop. A fluid is circulated through the beds in one direction, a feed stock and a desorbent are introduced into the system, and at the same time, a raffinate (nonadsorbed component) and a sorbate (adsorbed component) are withdrawn from the system. The positions of introduction and withdrawal of these four kinds of fluid (hereunder referred to as side streams) are periodically shifted in the direction of the circulating fluid flow to effect chromatographic separation of the respective components. Several proposed methods for controlling the operation of this simulated moving bed system are described in Japanese Patent Publication No. 18392/76 and Japanese non-examined Publications (OPI) Nos. 88274/78 (corresponding to U.S. Pat. No. 4,182,633) and 90072/79.
In the operation of the simulated moving bed system, the flow rates of three of the four side streams are usually controlled by setting them to predetermined values, whereas the other side stream is controlled by a control valve that maintains the pressure in the bed at a constant level. When only one circulating pump is positioned in the conduits, the suction pressure of the pump provides the lowest pressure in the system, and the last of the above-mentioned side stream is usually controlled so that the suction pressure of the circulating pump is held constant. The function of the circulating pump is to compensate for the pressure drop occurring in the beds. Therefore, when only one circulating pump is used, its discharge pressure is equal to the sum of its suction pressure and the pressure drop in the beds, and thus the beds must be designated for pressure vessels that withstand this discharge pressure. This means a high construction cost is incurred if the pressure drop in the beds is to be great.
Theoretically, the pressure drop in the beds can be reduced by decreasing the ratio of the length of its straight section to its diameter (generally referred to as the L/D ratio of the bed). However, this is not a completely satisfactory solution in practice since a large-diameter bed presents difficulties in providing a uniform fluid flow and requires a large cost and installation area.
In order to reduce the designed pressure of the beds, several unit beds are arranged in series, connected by conduits to form an endless loop and equipped with two or more circulating pumps in the conduits in such a manner that the pressure drop occurring in the beds is distributed among the respective pumps.
However, in the simulated moving bed system, the respective components in the circulating fluid provide a given concentration distribution, and in the particular case of separation of a mixture of saccharides, the respective components have different viscosities or specific densities according to their concentration distribution. This means the pressure drop differs from one unit bed to another and that the pressure distribution among the unit beds tends to vary over time. Therefore, the suction pressure and discharge pressure of a given circulating pump will experience periodic variations. Furthermore, side streams are introduced into or withdrawn from the system so that the fluid flow varies from one zone of the bed to another. The control of the simulated moving bed system is further complicated by the fact that zones having different flow rates of the circulating fluid are shifted periodically.
The present inventors previously disclosed, in Japanese non-examined Publication No. 90072/79, a method for controlling a simulated moving bed system. This method depends on the finding that peak pressure in the system can be reduced by controlling the side streams at each phase of the operation in such a manner that the minimum or maximum value of the suction pressure or discharge pressure of the circulating pumps in the system is held constant.